Electrolytic production of silver chloride



i atented Sept. 5, 1 950 ELECTRQLYTIC PRODUCTION OF SILVER.

CHLORIDE I John B. Mullen, Chicago, Ill., assignor, by inesn'e assignments, to Burgess Battery Company, Freep'o'rt, 111., a corporation ofDelaware No Drawing. Application J ul-y 9, 1947,

Serial No. 759,911

7 Claims. (Cl. 204-56) This invention relates to an improved method for forming silver chloride electrolytically, and particularly to a method of electrolytically forming an adherent coating of silver chloride upon a body of silver.

In a known type of primary cell, one of the electrodes is composed of silver and has a layer of silver chloride applied thereto, which layer serves as the depolarizer of the cell. In a cell structure which has been used satisfactorily, the silver electrode is in the form of a thin sheet having the thickness of foil, and the layer oi silver chloride is formed in situ upon the silver sheet by electrolytic action. Such method of forming the layer of silver chloride has advantages in that it adheres well to the silver, is compact and exhibits low electrical resistance during cell action. It is to an improvement in the method of forming such layer of silver chloride that the present invention is directed.

The layer is formed by providing an electrolytic cell in which the sheet of silver is arranged as the positive electrode, or anode, in a bath of electrolyte comprising a dilute aqueous solution of a soluble chloride, and passing electric current through the cell. At the surface exposed to the electrolyte, the silver electrode undergoes anodic oxidation to silver chloride, which forms as a dense, compact layer in adherent relation to the surface of the silver sheet.

lhe method described has advantages, as previously mentioned, but also has the disadvantage that the silver sheet is not uniformly attacked, with the result that the layer of silver chloride is of non-uniform thickness and the thin silver sheet is frequently penetrated, that is, completely converted to silver chloride, at local areas before the silver chloride layer as a whole has been built up to the desired thickness. When the sheet is subsequently used in a primary cell, such local areas represent a complete waste of silver chloride because that portion of the electrode is inactive with respect to the function and operation of the cell. It is not unusual to employ a foil which initially has a thickness of only .003 inch and convert two-thirds of the silver to silver chloride, so that the importance of preventing penetration is apparent.

It is the object of the invention to provide an improved method of forming silver chloride by anodic oxidation of a body of silver, in which the oxidation reaction proceeds at a uniform rate over the area of the body of silver, and when the latter is in the form of a thin sheet, there is no objectionable local penetration.

I like.

It is an additional object of the invention to provide a method of the character described in which the formed layer of silver chloride is of uniform thickness.

It is a further object of the invention to provide an improved method of anodically oxidizing silver to form silver chloride in which the efiiciency of silver chloride formation is increased.

Briefly, the objects are accomplished by carrying out the anodic oxidation in a cell in which the electrolyte is a dilute aqueous solution of a soluble chloride having an alcohol such as methanol dissolved therein.

As an example of a satisfactory method in accordance with the invention, an electrolytic cell is made up having an electrolyte of the following composition: a

Pounds Water s Sodium chloride 6 Hydrochloric acid (38% solution in water) 1 Methanol The above solution has a pH of approximately 1.0. The composition and pH are maintained approximately as given by suitable replenishments of the various ingredients as they are consumed during operation of the cell. The cell is completed by arranging the electrodes therein, one of which is composed of silver and the other of which may be composed of any conductive composition which is not attacked by the electrolyte, such as carbon, silver, gold, platinum, etc., and the electrodes are connected to a source of direct current energy in such manner that the silver electrode is the positive electrode, or anode. Any suitable voltage and current density may be used, for example, 2 volts and 12 amperes per square foot of anode surface. The voltage usually must be increased as operation progresses in order to maintain the current at a constant value. As stated heretofore, if the silver chloride coated sheet is to be used subsequently in a primary cell, the silver body may be in the form of a thin sheet or foil. It may be desired that the formation of silver chloride take place on both sides of the sheet or upon one side only. If the latter is the case, the cell may be arranged so that the action is confined to one side of the sheet, as by applying a suitable resistant composition to the surface at which anodic oxidation of the silver is to be prevented, such as varnish, wax, the copolymer of vinyl chloride and vinyl acetate in a suitable solvent, or the During the electrolytic action, the electrolyte bath is maintained at a temperature between approximately 40 C. and 60 C.

It has been found that when the method is carried out in accordance with the above example, a layer of silver chloride is formed upon the surface of the silver anode and that. the action progresses: with surprising uniformity over the area of the anode, such that the silver chloride continues to build up in the form of a layer of substantially uniform thickness during the operation of the cell, and the sheet of silver becomes progressively and uniformly thinner throughout its area and is not penetrated. or completely converted to silver chloride at any local areas thereof. The electrolytic action may be discontinued when the desired proportion of the silver has been converted tosilver chloride;

In addition to the advantage of uniformity of anode oxidation, the use of methanol inthe electrolyte results in an increase in the anode efficiency ofthe electrolytic cell. For example, when the method is carried out in accordance with the example given heretofore, that is, em.- ploying a temperature of from 40 C. to 60 C.

and an electrolyte in which. the liquid constituent is composed of- 5% methanol and 95% water, the same current input produces a deposit of silver chloride which is greater by approximately than is produced under similar circumstances but without the use of methanol in: the electrolyte, that is, with a liquid constituent which is 100% water. In terms of anode efficiency, with an electrolyte in which the solvent is100% water, the anode efliciency is approximately 85% to 95%, while when 5% or more of methanol is used, the anode eificiency is increased to substantially 100%.

While the above is given as an example of a satisfactory method in accordance with, the invention, many variations may be made therein while preserving the desired results. For example, the proportion of methanol may be varied. In general, the uniformity of anodic oxidation is improved as the concentrationof methanol is. increased, and the proportion: of methanol may be increased until. the solution. contains only suflicient water and conductive salt to: impart the desired conductivity. Insuchcase, the liquid constituent of the solution may. contain as much as 95% by" weight of methanol and as little as 5% of water. To incorporate the desired: amount of sodium chloride, such a soldtion. becomes a substantially saturated solution of. sodiumchloride. The useof suchiasolution provides the desired. uniformity of. anodic oxidation, but the conductivity. of the cell is less and. the operation is more. expensive than the operation. of. the cell employing a lower proportion of methanol. Usually, aconcentration of 5% of methanol, as is given in the foregoing example, produces satisfactory results, and'the'use of more than 10% of methanol isundesirable for economical reasons.

The temperature of the electrolyte. may also be varied. In general, the uniformity'of'anodic oxidation is improved as the temperature is increased-,but it is preferred-not to-employ a temperature above 60 C., because obj'ectionableloss of: methanol by vaporization occurs above that temperature.

There is, therefore, a correlation between the factors of temperature and methanolconcentration. With higher concentrations of methanol, a lower temperature may be used. with equally good results. For example, with a temperature of 25 C. and an electrolyte in which the liquid constituent is composed of 20% methanol and water, a uniformity is obtained which is substantially equal to that obtained with a temperature of 50 C. and a liquid constituent of 5% methanol and water.

The water soluble alcohols other than methanol may be employed to improve the uniformity of anodic oxidation. Ethanol is an example of another alcohol and an electrolyte composition similar to the example given heretofore, in which 5 pounds of ethanol is substituted for the 5 pounds of methanol, has been found to produce a satisfactory reduction in the local penetration of the silver sheet.

Salts other than sodium chloride may be used. Any other soluble chloride, such as potassium chloride may be used, and hydrochloric acid may be used in the absence of a chloride salt. For best results, the pH of the electrolyte should be maintained between approximately 1 and approximately 3. The soluble chloride and hydrochloric acid provide the desired. conductivity and pH and also the supply of chloride ions for the anodic oxidation function.

What is claimed is:

1. In anodically coating silver chloride upon a silver body in an acidicv aqueous electrolyte containing a water-soluble chloride, the improvement which consists in including in the electrolyte a substantial amount of an alcohol from the group consisting of methanol and ethanol.

2'. The method as claimed in claim 1. in which the proportion of alcohol is approximately 5% to 95% by weight of the liquid constituents of the electrolyte.

3. The method as claimed in claim 1 in which the pH of the bath is maintained at approximately 1 to 3.

4. The method as claimed in claim 1, in which the temperature of the bath is maintained be tween approximately 40 C. and approximately 60C.

5. The method as claimed in claim 1 in which the body of silver is in the form of a sheet.

6. In anodically coating silver chloride upon a silver body in an acidic aqueous electrolyte containing a water-soluble chloride, the improvement which consists in including in the electrolyte a substantial amount of methanol.

'7. The method as claimed in claim 6' in which the amount of methanol is maintained at not more than approximately 10% of the weight of the liquid constituents of the electrolyte.

JOHN B. MULLEN.

Axtell May 15, 1917 Lawson Oct. 14, 1947 FOREIGN PATENTS Country State Great Britain Mar. 13, 1919 OTHER REFERENCES Jahn: Z. Phy. Chem., vol. 33 (1900), p. 556.

Moore et al.: Chemical Abstracts, vol; 41. (1947), p.5031'.

Jour. Amer. Chem.

Number Number Soc, vol. 199), pp. 

1. IN ANODICALLY COATING SILVER CHLORIDE UPON A SILVER BODY IN AN ACIDIC AQUEOUS ELECTROLYTE CONTAINING A WATER-SOLUBLE CHLORIDE, THE IMPROVEMENT WHICH CONSISTS IN INCLUDING IN THE ELECTROLYTE A SUBSTANTIAL AMOUNT OF AN ALCOHOL FROM THE GROUP CONSISTING OF METHANOL AND ETHANOL. 